A better question is how fast can Kurama generate chakara?
as quick as the plot demands
Hermit mode + Kyubi = quickly recovering naruto
Perhaps you discounting the time when every one was chatting Naruto and Kurama was actually recovering ....
Naruto's chakra was complete sucked but not kurama. Kurama has too much chakra. So he gave it to Naruto and Naruto mixed Sage Mode again
In that case naruto should'nt ve lost his BM earlier
But the speed is just too much........naruto looked totally drained in the previous chapter when hiruzen saved him....but he recovered completely in a few frames.......compared to the last time...this was just too fast